Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 12,385.9	$ 11,984.1	$ 12,007.8
Net Investment Gain (Loss)	(36.0)	(15.7)	76.7
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1,640.1	1,750.0	1,260.6
Operating Lease, Impairment Loss			(13.9)
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)			67.6
Amortization of the Cost of Reinsurance	(44.1)	(50.3)	(69.8)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	12,385.9	11,984.1	12,007.8
Net Investment Gain (Loss)	(36.0)	(15.7)	76.7
Adjusted Operating Revenue	12,421.9	11,999.8	11,931.1
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1,640.1	1,750.0	1,260.6
Net Reserve Decrease Related to Reserve Assumption Updates, Before Tax	(177.2)	243.3	235.0
Capitalized Computer Software, Impairments	0.0	0.0	(12.1)
Costs Related to Early Retirement of Debt	0.0	0.0	(67.3)
Operating Lease, Impairment Loss	0.0	0.0	(13.9)
Adjusted Operating Income	1,932.2	1,607.1	1,151.1
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	0.0	0.0	67.6
Amortization of the Cost of Reinsurance		(50.3)
Non-Contemporaneous Reinsurance	(34.8)	(34.4)	(32.9)
Transaction Costs	0.0	0.0	(6.2)
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	$ (36.0)	$ (15.7)	$ 9.1